I've just played a chunk of one of the combat missions in the new Mass Effect 3 DLC, Leviathan. There was a boat. There was water. There was a mech. And Shepard dove to the bottom of the sea, Abyss-style.

Before I got the controller, BioWare's Nick Clifford took us through a cinematic near the start of the DLC. Shepard was aboard the Citadel, interrogating a member of a research team that was looking into something called "The Leviathan," which is a mysterious being apparently capable of killing a Reaper. (I'm also guessing it's quite large, going by the name…)

Vital stats, according to Clifford:

Leviathan will be 2 to 3 hours long, probably closer to 3 hours. Clifford said they were shooting for about a Lair of the Shadow Broker depth.

The story will take place in the middle of the Mass Effect 3 story, with no specific place in the storyline.

There are 3 main combat missions, which will take place all over the Mass Effect universe.

There will be new weapons, some of which were previously pre-order bonuses.

The story begins on the Citadel, but goes beyond that. (All the way to the bottom of the ocean. Well, an ocean.)

It will be out on all platforms simultaneously.

It will be out this summer, after the Firefight weapon pack, which comes out August 7 Th..

It will cost $9.99.

We then jumped ahead to the third and final combat mission in the DLC, which was when I took over. Shepard and his crew were on the deck of an abandoned ship floating in the middle of a windswept (and gorgeous) ocean. The ship pitched about the waters as I played, giving the environment a nice (or nauseating) sense of motion. You know, if you're into that sort of thing.

Shepard's team had crash-landed on the deck, and the pilot Cortez was attempting to fix the Normandy's shuttle. I had to fend off wave after wave of attacking Reaper foot soldiers—it would have been a pretty stiff battle, with several brutes dropping in a row, but the game had been set to easy for demoing purposes, so it was a cinch.

My demo Shepard was a soldier, so I whipped out some incendiary ammo and knifed through the reapers. There are a bunch of new weapons in the DLC, in addition to the Firefight DLC weapon pack. Most of the new weapons were already available as various pre-order bonuses, though at least two of the weapons in the firefight pack are new.

After making mincemeat of the reaper forces, Shepard and Cortez put their heads together to figure out how best to track down the Leviathan who was, it looked like, at the bottom of the ocean. (It was a bit hard to make out all of the dialogue, as there were a bunch of other games being played in the room with me.)

Cortez and Shepard realized that there was a combat mech on the ship that could go underwater, but we'd have to power up the doors blocking the way to it. So, I put a battery pack on my back and fought my way across the deck to a charging station, where I hit a button to begin charging up the door. Clifford said that multiplayer has let the folks at BioWare had learned a lot about making combat interesting by watching the Mass Effect 3 multiplayer, and that they were much more interested in creating objectives like this on the battlefield.

I fought my way back to Cortez to get another battery, then back to the charging station, then one more time, before the mech was finally available. Shepard hopped in, and the crew was overrun with enemies. Then came the humorous mission objective:

KILL ALL ENEMIES

Ha. So I completed that objective with gusto, fighting from a first-person perspective like the mech sequence in the main game. After that, I went and checked back in with Cortez, before we came to the conclusion that it was time to take this mech underwater.

Shepard walked up to the side of the deck, looked down into the water, and jumped. A cool cutscene unfolded, with Shepard's mech floating down towards the bottom of the ocean. It was one of those iconic sci fi scenes, like Sphere or The Abyss—a lone submersible tumbling through the dark, a ray of light going only so far into the murky depths.

Clifford said that they were excited to be exploring these new sorts of experiences in Mass Effect—the isolation at the bottom of the ocean, Shepard without his team. It showed—this really did feel like a new experience in Mass Effect. Then again, it comes at the end of the final mission, so this sequence really did seem like the high point of the DLC.

Shepard's mech landed on the sea floor, huge shapes looming in the distance, and he shot a flare onto the ocean floor. The sea life looked colorful and very BiosShock ian. I took control of the mech, and its heads-up display had changed to register air pressure, and it looked like the weapons were now offline. I walked forward a bit, at which point the demo ended; clearly things were about to get more interesting.